Distillate fuels produced from Fischer Tropsch products (i.e., waxes and condensates) by hydroprocessing (hydrotreating, hydrocracking, hydroisomerization, and related processes) have excellent cetane numbers, and very low sulfur and aromatic content. These properties make Fischer Tropsch products generally suitable for use as a fuel where environmental concerns are important. However, due to their high paraffin and low aromatic contents, Fischer-Tropsch distillate fuels have certain properties that are problematic when used as a fuel.
By way of example, Fischer-Tropsch distillate fuels have problems with poor seal swell properties. The seal swell problems associated with Fischer-Tropsch distillate fuel components may limit their use.
The impact of lowering the aromatic content of distillate fuels used as diesel fuel or jet fuel on seal swell in diesel and jet engines is known, and became important when California switched from conventional diesel fuel to Low Aromatics Diesel Fuel (LAD). LAD does not contain zero aromatics, but must contain less than 10%. Literature references related to the problems encountered with lowering the aromatic content of distillate fuels include: Transport Topics, National Newspaper of the Trucking Industry, Alexandria, Va., “Fuel Pump Leaks Tied to Low Sulfur,” Oct. 11, 1993; Oil Express, “EPA's diesel rules leading to shortages, fleet problems, price hikes,” Oct. 11, 1993, p 4; Marin Independent Journal, “Motorists in Marin angry over fuel change,” Nov. 11, 1993, p A1; San Jose Mercury News, “Mechanics finger new diesel fuel,” Dec. 3, 1993; and San Francisco Chronicle, “Problems With New Diesel Fuel, Clean Air, Angry California Drivers,” Dec. 23, 1993.
The problem of poor seal swell may be monitored by measuring the swelling of gaskets. The swelling of gaskets can be monitored by the use of known tests. For example, a description of test methodology is presented in SAE Paper No. 942018, “Effect of Automotive Gas Oil Composition on Elastomer Behavior,” October 1994, which describes seal swell and hardness changes which were measured in test procedures, based as closely as possible, on a British Standard (BS) method BS 903 Part A 16 [British Standard Institute, ‘Methods for testing vulcanized rubber,’ Part A 16:1987—Determination of the effect of liquids], which is broadly similar to American Society for Testing and Materials (ASTM) procedures D471 [Test Method for Rubber Property-Effect of Liquids] and D2240 [“Test Method for Rubber Property-Durometer Hardness].” (See FIG. 12). The paper examines volume swelling of five types of elastomers: hydrogenated nitrile, low nitrile, medium nitrile and low nitrile rubbers, and fluorocarbon elastomer.
A summary of work carried out to assess problems associated with California low sulfur/low aromatics fuels is presented in the California Governor's “Diesel Fuel Task Force Final Report,” dated Mar. 29, 1996. The report notes results of measurements carried out on O-rings before and after immersion in fuels: volume and weight change by ASTM D471 [Test Method for Rubber Property-Effect of Liquids], hardness by ASTM D1415 [Test Method for Rubber Property-International Hardness], and modulus of elasticity, ultimate tensile strength and elongation by ASTM D1414 [Test Methods for Rubber O-Rings].
In addition to problems with seal swell, highly paraffinic fuels have low densities in comparison to specifications of standard fuels. Lower densities are an important concern with jet fuels because jet fuels with low densities give decreased range of flight. In addition, with diesel fuels low densities are expected to give low driving ranges and likely lead to customer dissatisfaction. The following Table I summarizes the densities of some paraffins in the distillate boiling range.
TABLE IDensities of Paraffins in the Distillate Boiling RangeDensity,Net Heat ofNet Heat ofCetaneg/cm3 atCombustion,Combustion,ParaffinNo.20° C.BTU/GalMJ/kgn-ParaffinsDecane (n-C10H22)760.7301115,88044.25n-Pentadecane950.7684121,25043.99(n-C15H32)n-hexadecane (n-C16H34)100 0.7735122,00043.95Eicosane (n-C20H42)110 0.7843123,44043.87i-Paraffins2-methylheptane0.6979111,11044.38(i-C8H18)2,2-Dimethyloctane0.7245114,75044.16(i-C10H22)2-methyludecane0.7475117,90044.08(i-C12H26)Aromaticsp-xylene (C8H10)0.8610128,90040.81n-nonylbenzene (C15H24)500.8558129,41042.15n-decylbenzene (C16H26)0.8554129,60043.95n-tetradecylbenzene720.8549130,31042.50(C20H34)
According to ASTM D 1655 specifications for jet fuel, the range of acceptable densities at 15° C. for Jet A and Jet A-1 is 775-840 kg/m3 (0.775 to 0.840 g/cm3). Thus, pure n-paraffins appear to have unacceptably low densities. Isomerization of the paraffins, critical to meeting cold climate specifications such as pour, cloud and freeze points, often slightly lowers their densities even further making their fit with the minimum requirements of acceptable densities even poorer. Adjusting of the above listed densities for a temperature of 15° C. will typically only increase the densities by 0.004 g/cm3; therefore, these conclusions will not be changed significantly. Lower densities are an important concern because jet fuels with low densities give decreased range of flight.
In the United States, ASTM D975 sets the specifications for diesel fuel; however, it does not include specifications for density or energy content of diesel fuel. However, as noted in “Technical Review, Diesel Fuels, Chevron Products Company,” (FTR-2, 1998), Page 31, typical low sulfur fuels have relative densities between 0.83 and 0.86 g/cm3 at 15° C., and typical net heating contents of 130,000 Btu/Gallon. Corresponding values for paraffins (both normal and iso) are below these cited typical values. Thus, highly paraffinic diesel fuels are expected to give low driving ranges and likely lead to customer dissatisfaction.
The National Conference on Weights and Measures (NCSM) provides a definition for “premium diesel fuel.” (“Technical Review, Diesel Fuels, Chevron Products Company,” (FTR-2, 1998), Pages 35-36). Part of the specifications for this premium diesel fuel includes a minimum gross energy content of 138,700 Btu/gal, which is equivalent to a minimum net energy content of 130,500 Btu/gal. Aromatics come closest to this limit for minimum gross energy. Therefore, pure paraffinic diesel fuels will have densities and energy contents below the typical ranges of fuels, and below the emerging specifications for premium fuels.
An additional problem associated with highly paraffinic distillate fuels is that paraffins can have unacceptably low viscosities, which is another important property of distillate fuels.
Fischer Tropsch-derived distillate fuels also have known problems of poor lubricity, as explained in U.S. Pat. Nos. 5,689,031; 5,766,274; 6,017,372; 6,274,029; 6,296,757; 6,309,432; and 6,607,568. The solution proposed in these patents is typically to blend a hydrotreated Fischer Tropsch product with portions of the Fischer Tropsch product that have not been hydrotreated.
However, unless all components of the Fischer Tropsch-derived distillate are hydrotreated, the blend of Fischer Tropsch products can rapidly form peroxides as described in copending U.S. Patent Publication Nos. 20040152930 and 20040148850.
In contrast, distillate fuels produced from petroleum products often have high sulfur and aromatic levels, but have good or exceptional densities and volumetric energy contents. The high sulfur levels can be reduced by hydroprocessing, but hydroprocessing can result in reduction in aromatics to the extent that problems with seal swell, density, or volumetric energy content emerge. In addition, it has been found that hydrotreated petroleum stocks can have stability problems due to the formation of peroxides.
It may be desirable to produce blends of Fischer Tropsch-derived and petroleum-derived distillate fuels in an attempt to meet density and energy specifications. However, as described in U.S. Pat. No. 6,776,897, blends of Fischer Tropsch-derived and petroleum-derived distillate fuels can have poor stability in ASTM D6468 diesel test, which measures distillate fuel thermal stability, and as described in U.S. Patent Publication No. 20030116469 blends of Fischer Tropsch-derived and petroleum-derived distillate fuels can have poor stability in ASTM D3241, which measures thermal stability of jet fuels. An additional problem associated with highly paraffinic distillate fuels is that paraffins can have unacceptably low viscosities, which is another important property of distillate fuels.
An additional problem associated with hydroprocessing the highly paraffinic distillate fuels and/or the petroleum-derived distillate fuels is that hydroprocessing of these fuels consumes hydrogen. The hydrogen needed for these upgrading processes can be expensive to produce and to store, if necessary. Therefore, it would be desirable to minimize the need for this hydrogen.
Accordingly, there is a need in the art for distillate fuels with acceptable seal swell properties. There is further a need in the art for distillate fuels with satisfactory density properties. Finally, there is a need in the art for distillate fuels with satisfactory properties that can be obtained, at least in part, from Fischer-Tropsch process products. This invention provides such distillate fuels and the processes for their manufacture.